Dry flue gas desulfurization (DFGD) covers a range of technologies that include the absorption of sulfur dioxide (SO2) by reaction with alkaline materials. These materials can be dry particles, dry particles in a humid environment, or moist particles in the process of drying. The absorption of SO2 with moist particles is typically accomplished using a spray dryer.
Spray drying is a DFGD process where a lime slurry is prepared by slaking lime. The slaked lime is usually stored in a tank and then transferred to the spray dryer. A spray dryer is a large, empty vessel where flue gas is directed at high velocity toward lime slurry atomizers. The atomizer uses high energy to divide the slurry into extremely fine droplets and dispense them into the high velocity gas stream. The result is three activities that occur at essentially the same time, albeit incompletely. Sulfur dioxide is absorbed into the droplets, the sulfur dioxide reacts with the lime to form a mixture of calcium sulfite and calcium sulfate, and the droplet dries leaving the reaction products suspended in the gas. Historically, the resulting particulates are carried out of the spray dryer with the flue gas for separation and disposal.
DFGD systems that only use fresh lime (often referred to a “once-through lime”) use a large quantity of lime as compared to DFGD systems that recycle at least a portion of lime already used in the system. However, known lime recycle systems are expensive and time consuming because of the need to combine the used lime with dilution water. A system that reduces fresh lime consumption while also reducing cost and time in lime recycling is desired.